Interpretive Handbook

Alveolar rhabdomyosarcoma (ARMS) is a member of the family of rhabdomyosarcomas (tumors composed of cells showing muscle differentiation) that also includes embryonal, botryoid, spindle cell, and pleomorphic types.(1) Alveolar rhabdomyosarcomas include the classical and solid patterns.(1) ARMS is also a member the small-round-cell tumor group that includes synovial sarcoma, lymphoma, Wilms tumor, Ewing sarcoma, and desmoplastic small-round-cell tumor.

While treatment and prognosis depend on establishing the correct diagnosis, the diagnosis of sarcomas that form the small-round-cell tumor group can be very difficult by light microscopic examination alone, especially true when only small-needle biopsy specimens are available for examination. The use of immunohistochemical stains (eg, desmin, actin, and the nuclear transcription factor markers MyoD and myogenin) are useful in separating rhabdomyosarcomas from other small-round-cell tumors, but do not always distinguish the various subtypes of rhabdomyosarcomas. Expertise in soft tissue and bone pathology are often needed.

Studies have shown that some sarcomas have specific recurrent chromosomal translocations. These translocations produce highly specific gene fusions that help define and characterize subtypes of sarcomas and are useful in the diagnosis of these lesions.(1-4)

Most cases of ARMS have a t(2;13)(q35;q14) reciprocal translocation. This rearrangement juxtaposes 5' portions of the PAX3 gene on chromosome 2 with 3' portion of the FOXO1A gene on chromosome 13 resulting in a chimeric gene in the designated chromosome 13 that encodes a transcriptional regulatory protein in 75% of cases.(1) Another variant t(1;13)(q36;q14) translocation fuses the 5' portion of the PAX7 gene on chromosome 1 with the FOXO1A gene on chromosome 13 in a smaller number of cases (10%). The PAX3-FOXO1A fusion is associated with a worse prognosis than the PAX7-FOXO1A fusion.

Supporting the diagnosis of alveolar rhabdomyosarcoma

A positive result is consistent with a diagnosis of alveolar rhabdomyosarcoma (ARMS).

Sarcomas other than ARMS, and carcinomas, melanomas, and lymphomas are negative for the fusion products.

A negative result does not rule-out a diagnosis of ARMS.

Reliable results are dependent on adequate specimen collection and processing. This test has been validated on formalin-fixed, paraffin-embedded tissues; other types of fixatives are discouraged. Improper treatment of tissues, such as decalcification, may cause PCR failure.

Clinical diagnosis and therapy should not be based solely on this assay. The results should be considered in conjunction with clinical information, histologic evaluation, and additional diagnostic tests.

Negative

1. Edwards RH, Chatten J, Xiong QB, Barr FG: Detection of gene fusions in rhabdomyosarcoma by reverse transcriptase polymerase chain reaction assay of archival samples. Diagn Mol Pathol 1997;6:91-97 

2. Ladanyi M, Bridge JA: Contribution of molecular genetic data to the classification of sarcomas. Hum Pathol 2000;31:532-538 

3. Galili N, Davis RJ, Fredricks WJ, et al: Fusion of a fork head domain gene to PAX3 in the solid tumor alveolar rhabdomyosarcoma. Nat Genet 1993;5:230-235 

4. Jin L, Majerus J, Oliveira A, et al: Detection of fusion gene transcripts in fresh-frozen and formalin-fixed paraffin-embedded tissue sections of soft tissue sarcomas after laser capture microdissection and RT-PCR. Diagn Mol Pathol 2003;12:224-230